1. Field of the Invention
The present invention relates to a process for thermal debinding and sintering of a workpiece.
2. Description of the Background
Injection molding is a widely used method for fabricating specimens of complex geometry with precise dimensions, high density and homogenous microstructure. This process entails injecting a powder-polymer mixture into a mold, and heating the mixture to a temperature where its viscosity is low enough to allow flow and then injecting the mixture into a cold area or cold containment means where it solidifies into desire shape. After molding, the binder is removed and the powder network which remains is sintered. In view of the high volemic fraction of the binder, 40 to 60 vol %, the removal of the binder from the parts, or debinding, is a critical step in the process.
At present, producers are limited to the use of small parts because debinding of thick sections requires a long time and often leads to defects such as distortion, cracking or blistering. Besides those problems, producers are also confronted with the control of the chemical composition of the products, mostly carbon and oxygen content, which are affected by the polymer decomposition.
Numerous debinding techniques have been developed, such as thermal debinding and solvent debinding. Thermal debinding effects binder removal by increasing the temperature. The polymer is decomposed into volatile species that diffuse through the compact to the surface, where it is removed. In this process, a high production rate of volatile species must be avoided to prevent defect formation within the green body. Thus, a slow removal rate is required, and consequently a long debinding cycle. This is the principal drawback of this debinding technique. Another example of thermal debinding is wicking debinding. In this process, the parts are in contact with a porous material (powder bed) which provides capillary flow for the binder which becomes liquid as the temperature increases. Compared with thermal debinding, wicking debinding saves time and allows better shape retention. The principal drawback of this method is the numerous manipulations of the parts, i.e. placing the parts into the powder bed, then removing and cleaning them.
Solvent debinding, involves immersion of the parts into a solvent which dissolves the polymer. The time of debinding may be reduced considerably, sometimes from days to a few hours, with better control of distortion than with thermal debinding. Nevertheless, because of the cost and toxicity of the solvents, this method is rarely used in industry.
Even though thermal debinding requires a long time and may be a source of numerous product defects, its simplicity and relative safety have favored its use in industry. Three theoretical steps may be used to describe the pyrolytical removal of binder from a green body by thermal degradation. Evaporation is only possible for species with low molecular weight, which undergo evaporation without chain scission. By low molecular weight is generally meant below about 1000 g/mol. The process of oxidation originates from an initial bond breaking reaction such as C--C bond scission. The following equations describe the oxidative degradation of a polymer. ##EQU1##
Thermal degradation of a polymer occurs either by random scission through the polymer carbon backbone, or by monomer split-off.
The role of atmosphere during thermal debinding is fundamental. The use of gas compositions, such as nitrogen or argon, lead to a thermal degradation of the binder (random scission or/and monomer split-off). In this case, thermal degradation may occur in the core as well as in the surface of the part. Thus, the risk of defect formation is high because of the possibility of pressure build up within the compacts.
Oxidative atmospheres are also used in industry. With air or various mixture of oxygen and nitrogen, the binder oxidation is limited by oxygen diffusion through the porosity of the compacts. At the beginning, the reaction is limited to the surface; then the interface of decomposition moves toward the part center. Oxidative atmospheres offer the advantage of a progressive debinding with a limited risk of defect formation. Moreover, oxidative degradation is an auto-catalytic process which consequently leads to a rapid binder degradation. The principal drawback of using an oxidative debinding atmosphere is the risk of oxidation of the powder. For materials such as stainless steel, the sensitivity of chromium to oxidation forbids the use of oxidative atmosphere during debinding. The same conclusion can be drawn for ceramic materials, such as nitrides or carbides.
As an example of thermal debinding under neutral atmospheres, the experiments of Renowden and Pourtalet may be noted. See, Renowden, M. and Pourtalet, P., "Experimental Studies on Lubricant Removal", 1990 Advances in Powder Metallurgy, vol. 1, pp. 261-277.
In this study, the effect of veritable atmospheric composition on removal of zinc stearate lubricant was conducted. Generally, this article describes that the important steps in removing lubricants are 1) heating the mixture until the temperature of binder vaporization is reached, 2) transferring the vapor lubricant from the inside to the surface, 3) removing the vapor lubricant from the surface and 4) burning off the lubricants.
From this study, it was only concluded that a preferred atmosphere is 30% H.sub.2, balance N.sub.2 to remove wax binder, and that 50% H.sub.2, balance N.sub.2 is necessary at 550.degree. C. to achieve 90% decomposition of the organic portion of zinc stearate lubricant. Further, it was found that at 550.degree. C., and in a dry atmosphere, the removal of binder is complete, although higher temperature and more H.sub.2 is necessary to remove zinc stearate lubricant. This process entails a slow decomposition of binder and poses a high risk of defect formation, particularly warping distortion.
Thus, in essence, thermal debinding under neutral atmospheres is limited by a slow binder decomposition process and a high risk of forming defects, while, thermal debinding under oxidative atmospheres is restricted to powders which are not sensitive to oxidation.
Thus, a need exists for a process of effecting thermal debinding of a workpiece under a neutral or oxidative atmosphere which is neither limited by a slow binder decomposition process nor a high risk of forming defects, particularly warping distortion.